Optimise default partition scanning while adding new partition

Hi,

Commit 6f6b99d1335be8ea1b74581fc489a97b109dd08a introduced default
partitioning support. This commit added a new function
check_default_allows_bound(),
which checks if there exists a row in the default partition that would
belong to
the new partition being added. If it finds one, it throws an error. Before
taking
the decision to scan the default partition, this function checks if there
are
existing constraints on default partition that would imply the new partition
constraints, if yes it skips scanning the default partition, otherwise it
scans the
default partition and its children(if any). But, while doing so the current
code
misses the fact that there can be constraints on the child of default
partition
such that they would imply the constraints of the new partition being added,
and hence individual child scan can also be skipped.
Attached is the patch which does this.

This is previously discussed in default partitioning thread[1], and decision
was made that we can take this a separate patch rather than as a part of the
default partitioning support.

Amit Langote has a similar patch[2] for scanning the children of a
partitioned
table which is being attached as partition of another partitioned table.

[1]
https://www.postgresql.org/message-id/CA+TgmoZ8-q=2OahOXmvZbDNxi9G6i1iDi4OZfkb67MK242D_Aw@mail.gmail.com
[2]
https://www.postgresql.org/message-id/4cd13b03-846d-dc65-89de-1fd9743a3869@lab.ntt.co.jp

Regards,
Jeevan Ladhe